Low temperature conductive coating has been proposed for some time but has never been applied in a large commercial scale because of its instability, likelihood of cracking at high temperature, and expensive manufacturing costs with high vacuum vapor deposition processes needed to achieve a uniform composition and structure. Development of a uniform composition and thickness as well as a stable structure across the entire conductive layer is critical to maintain a consistent resistance and temperature distribution of the heating element of the heating apparatus. Resistance variation across the conductive layer may create temperature variation/gradient and thus thermal stress in the conductive layer, which can de-stabilize the structure and cause cracking of the layer, particularly in high temperature heating applications.
PCT Publication No. WO00/18189 by Torpy et al., incorporated herein by reference, has proposed a coating system by doping tin oxides with cerium and lanthanum to increase the stability of the conductive film on a glass substrate for heating purposes. However cerium and lanthanum have to be uniformly distributed within the coating to provide a stabilizing effect, which is generally difficult to achieve. A one hour annealing at a high temperature has been proposed in PCT Publication No. WO00/18189 to help create a uniform and stabilized coating. However, it is not cost effective in manufacturing and may cause detrimental diffusion of contaminant elements from the substrate into the coating. Increasing the molar percentages of cerium and lanthanum may help in the distribution of these rare earth elements, but leads to increased electrical resistance of the film. This results in reduction of conductivity and power outputs, and imposes restrictions in practical and commercial use of the film.
The above description of the background is provided to aid in understanding the heating apparatus and the method of forming a heating element of a heating apparatus disclosed in the present application, but is not admitted to describe or constitute pertinent prior art to the heating apparatus and method disclosed in the present application, or consider the cited document as material to the patentability of the claims of the present application.